noUncheckedIndexedAccess with enums Type narrowed

src/compiler/checker.ts

@@ -15589,6 +15589,11 @@ m2: ${(this.mapper2 as unknown as DebugTypeMapper).__debugToString().split("\n")
                         return accessFlags & AccessFlags.IncludeUndefined ? getUnionType([indexInfo.type, undefinedType]) : indexInfo.type;
                     }
                     errorIfWritingToReadonlyIndex(indexInfo);
+                    if (accessFlags & AccessFlags.IncludeUndefined && objectType.symbol.flags & (SymbolFlags.RegularEnum | SymbolFlags.ConstEnum)) {
+                        if(indexType.flags & TypeFlags.EnumLiteral){
+                            return indexInfo.type;
+                        }
+                    }
                     return accessFlags & AccessFlags.IncludeUndefined ? getUnionType([indexInfo.type, undefinedType]) : indexInfo.type;
                 }
                 if (indexType.flags & TypeFlags.Never) {

tests/baselines/reference/noUncheckedIndexAccess.js

@@ -0,0 +1,37 @@
+//// [noUncheckedIndexAccess.ts]
+enum Meat {
+    Sausage,
+    Bacon
+  }
+  const sausage = Meat.Sausage
+  const valueSausage = Meat[sausage]
+
+  const bacon = Meat.Bacon
+  const valueBacon = Meat[bacon]
+
+  const union: Meat.Bacon | Meat.Sausage = Meat.Bacon
+  const valueUnion = Meat[union]
+
+  //Avoiding a false positive
+  const value = Meat[0]
+
+  const t = "testing"
+  const value2 = Meat[t]
+
+
+//// [noUncheckedIndexAccess.js]
+var Meat;
+(function (Meat) {
+    Meat[Meat["Sausage"] = 0] = "Sausage";
+    Meat[Meat["Bacon"] = 1] = "Bacon";
+})(Meat || (Meat = {}));
+var sausage = Meat.Sausage;
+var valueSausage = Meat[sausage];
+var bacon = Meat.Bacon;
+var valueBacon = Meat[bacon];
+var union = Meat.Bacon;
+var valueUnion = Meat[union];
+//Avoiding a false positive
+var value = Meat[0];
+var t = "testing";
+var value2 = Meat[t];

tests/baselines/reference/noUncheckedIndexAccess.symbols

@@ -0,0 +1,60 @@
+=== tests/cases/compiler/noUncheckedIndexAccess.ts ===
+enum Meat {
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+
+    Sausage,
+>Sausage : Symbol(Meat.Sausage, Decl(noUncheckedIndexAccess.ts, 0, 11))
+
+    Bacon
+>Bacon : Symbol(Meat.Bacon, Decl(noUncheckedIndexAccess.ts, 1, 12))
+  }
+  const sausage = Meat.Sausage
+>sausage : Symbol(sausage, Decl(noUncheckedIndexAccess.ts, 4, 7))
+>Meat.Sausage : Symbol(Meat.Sausage, Decl(noUncheckedIndexAccess.ts, 0, 11))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>Sausage : Symbol(Meat.Sausage, Decl(noUncheckedIndexAccess.ts, 0, 11))
+
+  const valueSausage = Meat[sausage]
+>valueSausage : Symbol(valueSausage, Decl(noUncheckedIndexAccess.ts, 5, 7))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>sausage : Symbol(sausage, Decl(noUncheckedIndexAccess.ts, 4, 7))
+
+  const bacon = Meat.Bacon
+>bacon : Symbol(bacon, Decl(noUncheckedIndexAccess.ts, 7, 7))
+>Meat.Bacon : Symbol(Meat.Bacon, Decl(noUncheckedIndexAccess.ts, 1, 12))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>Bacon : Symbol(Meat.Bacon, Decl(noUncheckedIndexAccess.ts, 1, 12))
+
+  const valueBacon = Meat[bacon]
+>valueBacon : Symbol(valueBacon, Decl(noUncheckedIndexAccess.ts, 8, 7))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>bacon : Symbol(bacon, Decl(noUncheckedIndexAccess.ts, 7, 7))
+
+  const union: Meat.Bacon | Meat.Sausage = Meat.Bacon
+>union : Symbol(union, Decl(noUncheckedIndexAccess.ts, 10, 7))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>Bacon : Symbol(Meat.Bacon, Decl(noUncheckedIndexAccess.ts, 1, 12))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>Sausage : Symbol(Meat.Sausage, Decl(noUncheckedIndexAccess.ts, 0, 11))
+>Meat.Bacon : Symbol(Meat.Bacon, Decl(noUncheckedIndexAccess.ts, 1, 12))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>Bacon : Symbol(Meat.Bacon, Decl(noUncheckedIndexAccess.ts, 1, 12))
+
+  const valueUnion = Meat[union]
+>valueUnion : Symbol(valueUnion, Decl(noUncheckedIndexAccess.ts, 11, 7))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>union : Symbol(union, Decl(noUncheckedIndexAccess.ts, 10, 7))
+
+  //Avoiding a false positive
+  const value = Meat[0]
+>value : Symbol(value, Decl(noUncheckedIndexAccess.ts, 14, 7))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+
+  const t = "testing"
+>t : Symbol(t, Decl(noUncheckedIndexAccess.ts, 16, 7))
+
+  const value2 = Meat[t]
+>value2 : Symbol(value2, Decl(noUncheckedIndexAccess.ts, 17, 7))
+>Meat : Symbol(Meat, Decl(noUncheckedIndexAccess.ts, 0, 0))
+>t : Symbol(t, Decl(noUncheckedIndexAccess.ts, 16, 7))
+

tests/baselines/reference/noUncheckedIndexAccess.types

@@ -0,0 +1,65 @@
+=== tests/cases/compiler/noUncheckedIndexAccess.ts ===
+enum Meat {
+>Meat : Meat
+
+    Sausage,
+>Sausage : Meat.Sausage
+
+    Bacon
+>Bacon : Meat.Bacon
+  }
+  const sausage = Meat.Sausage
+>sausage : Meat.Sausage
+>Meat.Sausage : Meat.Sausage
+>Meat : typeof Meat
+>Sausage : Meat.Sausage
+
+  const valueSausage = Meat[sausage]
+>valueSausage : string
+>Meat[sausage] : string
+>Meat : typeof Meat
+>sausage : Meat.Sausage
+
+  const bacon = Meat.Bacon
+>bacon : Meat.Bacon
+>Meat.Bacon : Meat.Bacon
+>Meat : typeof Meat
+>Bacon : Meat.Bacon
+
+  const valueBacon = Meat[bacon]
+>valueBacon : string
+>Meat[bacon] : string
+>Meat : typeof Meat
+>bacon : Meat.Bacon
+
+  const union: Meat.Bacon | Meat.Sausage = Meat.Bacon
+>union : Meat
+>Meat : any
+>Meat : any
+>Meat.Bacon : Meat.Bacon
+>Meat : typeof Meat
+>Bacon : Meat.Bacon
+
+  const valueUnion = Meat[union]
+>valueUnion : string
+>Meat[union] : string
+>Meat : typeof Meat
+>union : Meat.Bacon
+
+  //Avoiding a false positive
+  const value = Meat[0]
+>value : string | undefined
+>Meat[0] : string | undefined
+>Meat : typeof Meat
+>0 : 0
+
+  const t = "testing"
+>t : "testing"
+>"testing" : "testing"
+
+  const value2 = Meat[t]
+>value2 : error
+>Meat[t] : error
+>Meat : typeof Meat
+>t : "testing"
+

tests/cases/compiler/noUncheckedIndexAccess.ts

@@ -0,0 +1,21 @@
+//@noUncheckedIndexedAccess: true
+//@strictNullChecks: true
+
+enum Meat {
+    Sausage,
+    Bacon
+  }
+  const sausage = Meat.Sausage
+  const valueSausage = Meat[sausage]
+
+  const bacon = Meat.Bacon
+  const valueBacon = Meat[bacon]
+
+  const union: Meat.Bacon | Meat.Sausage = Meat.Bacon
+  const valueUnion = Meat[union]
+
+  //Avoiding a false positive
+  const value = Meat[0]
+
+  const t = "testing"
+  const value2 = Meat[t]